A loom control system comprising these features is already known from U.S. Pat. No. 4,326,564. In the art of weaving it is well known to provide an automatic loom with a loom control system operable to stop the loom in the presence of a yarn breakage. Automatic looms comprise a feeding device having a yarn storage drum for temporarily storing the yarn. Such feeding devices eliminate the wide variations in yarn tension which occur when a yarn is delivered from a supply source, and permit the yarn to be fed to the loom at a substantially constant tension, although the yarn is intermittently fed from the feeding device to the loom. Typically, such feeding device may either have a yarn storage drum upon which the yarn is wound as the drum is driven by an electric motor or the feeding device may incorporate a stationary yarn storage drum with an orbiting feeder tube driven by an electric motor and engaging the weft yarn to apply it to the surface of the stationary yarn storage drum. The feeding device includes a yarn store sensor which senses the quantity of yarn stored on the drum of the feeding device. The yarn store sensor generates an electric sensor signal representing said quantity of yarn. This signal is used for controlling the operation of the feeding device so as to control the quantity of yarn stored on the drum. Particularly, the sensor signal might be fed to a control unit which controls the operation of the feeding device by increasing or decreasing the rotary speed of the motor of the feeding device in such a manner so that the quantity of yarn stored on the drum essentially remains between a maximum quantity and a minimum quantity of yarn.
It is also known from the above-mentioned U.S. patent to equip loom control systems of the above-mentioned kind with a monitor means for stopping the loom in the presence of a yarn breakage. Such monitor means is necessary, as the weft yarn being conveyed from the yarn supply spool to the feeding device might break, which results in the weft yarn stored on the yarn storage drum ultimately becoming exhausted if the loom continues to operate. In that event, the insertion of the broken yarn will produce a defect in the woven fabric. Thus, it is desirable to stop the operation of the loom in case of a yarn breakage. The prior art monitor means comprise a tension device arranged between the yarn supply spool and the feeding device for sensing the yarn tension. The tension device generates a signal having a high logical potential if the yarn tension is above a predetermined value. If there is no yarn breakage, the output signal of the tension device is "high" during the feeding operation of the feeding device. If contrary hereto there occurs a yarn breakage between the yarn supply spool and the yarn storage feeder, the output signal of the tension device changes to zero potential. Thus, the occurence of a zero-signal during the operation of the feeding device is indicative of a yarn breakage, so that the simultaneous occurence of this zero-tension signal and a signal indicating the feeding device operation can be used for stopping the loom.
However, a yarn breakage might also occur between the yarn storage feeder and the loom. The prior art loom control system can only recognise this second kind of yarn breakage if it is equipped with a second tension device between the feeding device and the loom. Therefore, the prior art loom control system is undesirably complicated if arranged such that it is responsive to a yarn breakage before and after the feeding device. Furthermore, the prior art loom control system is not adapted to stop the loom in case of a malfunction of the feeding device, said malfunction either resulting in the case where the quantity of yarn stored on the drum exceeds a maximum quantity or resulting in the contrary case where the quantity of yarn falls below a predetermined minimum quantity. It is considered as being a further drawback of this prior art loom control system that its tension devices only have a quite unreliable operation as they are highly sensitive to dirt and dust which can prevent a correct operation of the tension device. Particularly, the free movability of the mechanically movable parts of the tension device can be blocked by dirt or dust. In addition, considerable wear occurs between yarn guiding eyelets of the tension device and the yarn. The wear increases in the case of an accumulation of dirt at the eyelets.
The task underlying the invention is to provide a loom control system in accordance with the generic clause of the main claim, which has a simple and cost-saving structure and which is reliable in operation.